1. Field of the Invention
The present invention relates to a holding check control valve used in hydraulic devices for heavy construction equipment.
2. Description of the Prior Art
Holding check control valves are used to restrain or slowly feed a flow of return fluid from an actuator to a tank for a certain time. Where such holding check control valves are used in heavy construction equipment such as excavators or cranes, they perform functions enabling the boom or the arm to hold a very heavy weight or to move downward without any impact while supporting the weight.
A conventional holding check control valve is illustrated in FIGS. 1A and 1B. The holding check control valve includes a main valve part and an assistant valve part which are shown in FIGS. 1A and 1B, respectively. As shown in FIG. 1A, the main valve part of holding check control valve includes a check valve 102 disposed in an actuator fluid passage 101. the check valve 102 has a spring chamber A in which a spring 106 is disposed. The spring 106 serves to always urge the check valve 102 toward its closed position. At an opened position of the check valve 102, a flow of return oil in the check valve 102 is allowed to return to a tank T via a return fluid passage 105. In order to pass the return oil in the actuator fluid passage 101 there through, the check valve 102 should move upward, when viewed in FIG. 1A, so that it is opened. When the fluid pressure in the spring chamber A of the check valve is reduced below the fluid pressure in the actuator fluid passage 101, the fluid pressure exerted on inclined surfaces 102a and 102b of the check valve 102 in the actuator fluidpassage 101 forces the check valve 102 to move upward against the resilience of the spring 106, thereby causing the check valve 102 to be opened. For reducing the fluid pressure in the spring chamber A, the fluid in the spring chamber A should be discharged. To this end, the assistant valve part of the holding check control valve is constructed to restrain the discharge of the fluid in the spring chamber A (thereby closing the check valve 102) or slowly discharge the fluid (thereby gradually opening the check valve 102). As shown in FIG. 1B, the assistant valve part of the holding check control valve includes a seat member 202 fitted in the interior of a body 201 of the holding check control valve. The seat member 202 has a hollow portion 203 in which a plunger 204 is disposed such that it can slide left or right. The plunger 204 is resiliently supported at one end thereof by a pressure setting spring 205 such that it is always urged in a left direction. The other end of the plunger 204 faces a piston 206 which is adapted to move in a right direction by a pilot pressure Pi. As the piston 206 moves in the right direction by the pilot pressure Pi, the plunger 204 is moved in the same direction, thereby causing the spring chamber B to communicate with the hollow portion 203 of seat member. 202. As a result, the spring chamber B communicates with a return fluid passage 207 connected to the tank T so that the fluid from the spring chamber B can return to the tank T via the hollow portion 203 and return fluid passage 207. Meanwhile, the spring chamber A of the main valve part and the spring chamber B of the assistant valve part communicate with each other via a communicating fluid passage 208. The return fluid passage 105 of the main valve part communicates with the return fluid passage 207 of the assistant valve part via another connecting fluid passage.
However, the conventional holding check control valve having the above-mentioned construction requires a considerably high pilot pressure for moving the piston 206 to move the plunger 204 against the high fluid pressure of the spring chamber A upon opening a fluid line associated therewith because the high fluid pressure of the spring chamber A is directly exerted on the spring chamber B. This results in the requirement of an additional high pilot pressure source. Since the return fluid passage 207 of the assistant valve part communicates with the tank via the return fluid passage 105 of the main valve part, the plunger 204 maybe unintentionally moved to open the associated fluid line when the fluid pressure in the spring chamber A, namely, the fluid pressure in the spring chamber B is lower than the fluid pressure in the tank T during an operation of the associated actuator, even if no pilot pressure is generated. In order to avoid such an undesirable phenomenon, the conventional holding check control valve requires an additional check valve installed in the return fluid Passage. Moreover, the conventional holding check control valve involves careful and precise machining work for obtaining accurate centering and straightness of the seat member and plunger and a high manufacturing cost.